Critical-Point Anomalies in the Electron-Paramagnetic-Resonance Linewidth and in the Zero-Field Relaxation Time of Antiferromagnets

A theory is presented for the anomalies in the electron-paramagnetic-resonance (EPR) linewidth and zero-field relaxation time of antiferromagnets. The analysis applies to the paramagnetic state immediately above the Néel point in systems where the dominant spin-spin interaction is the isotropic exchange coupling. It is assumed that the dipolar coupling is the principal source of anisotropy. The EPR linewidth and the relaxation rates for fluctuations in the total magnetization are separated into critical and noncritical parts; the latter are approximated by their values in the high-temperature limit. The anomalous increases in the linewidths and the relaxation rates are shown to arise from processes in which a fluctuation in the total magnetization decays into two fluctuations of the staggered magnetization via the dipolar coupling. The predictions of the theory are compared with linewidth measurements in RbMn${\mathrm{F}}_3$, Mn${\mathrm{F}}_2$, MnO, and MnS.